New York City Mayor Bill de Blasio has labelled the incident a clear “act of terror”.

The United States Secret Service has confirmed Mr Obama and Ms Clinton did not receive the packages.

“The packages were immediately identified during routine mail screening procedures as potential explosive devices and were appropriately handled as such,” the statement says.

“The protectees did not receive the packages nor were they at risk of receiving them.”

The incident comes just two weeks before nationwide elections that could reshape US Congress and serve as a referendum on the first two years of President Trump’s presidency.

President Donald Trump has condemned the attacks as despicable and confirmed a major federal investigation is underway.

The safety of the American People is my highest priority. I have just concluded a briefing with the FBI, Department of Justice, Department of Homeland Security, and the U.S. Secret Service… pic.twitter.com/nEUBcq4NOh